Barack Obama's announcement of an Afghan "surge" is his frantic bid to rescue what promises to be a stumbling re-election campaign that must start in 2011. It oozes with his desperation not to be in Afghanistan. The question is how best to disengage. As in Vietnam and as the Russians found, withdrawal tends to be possible here in Afghanistan only after the generals on the ground have been given a last chance to claim victory.

The chance is generous. With 30,000 more troops at a staggering cost of $1m per soldier per year, Obama's generals are charged with giving the Taliban a "knock-out" blow sufficient to send them reeling back into the mountains. This is supposed to allow the Kabul government to establish its sovereignty over its nation or, more plausibly, at least to give Nato a breathing space to escape.

This surge bears no relation to that in Iraq, except as an exit strategy. In Iraq it involved the intensive policing of the Baghdad suburbs plus the blatant recruitment of Saddam Hussein's old Sunni militias to keep the peace in their enclaves, despite the potential threat this posed to the al-Maliki government of Shias. It gave Baghdad's enclaves a measure of security and established a new, if tenuous, balance of power in the provinces. Above all, it took Iraq and its continued deaths and bombings out of the headlines.

In Afghanistan the strategy advanced by General Stanley McChrystal is not new. It involves flooding the towns with soldiers and money and hoping the Taliban will go away for the time being. The conditionals of army retraining and corruption eradication mean nothing. Afghan history says that "training" an Afghan army to fight Pashtun insurgents is futile. Afghans fight only for their tribe and its land, which is why the Taliban manages to train a ferocious soldier in days, while Nato has failed in years. Equally futile is to make withdrawal dependent on ridding the Kabul government of corruption. These conditions are just a smokescreen behind which Nato hopes to retreat. There is no more talk of 20-30 years. Obama needs to be leaving in 18 months.

If the Taliban commanders are wise, and they usually are in these matters, they will simply wait, controlling the country areas and killing Nato patrols with sufficient regularity to keep western public opinion demoralised. As the saying goes, Nato has the watches but the Taliban has the time.

Obama himself – and those round him – clearly has no stomach for this fight, any more than does Gordon Brown or the European allies. Afghanistan was a punitive raid that turned into an occupation that was not just mishandled but ill-conceived from the start. The operation now commencing is exit with dignity. Dignity will be the hard part.